An ink jet printer is an non-impact printing device that forms characters and other images by ejecting ink drops in a controllable way from a printhead. Ink jet printing mechanisms may be used in different devices such as printers, plotters, facsimile machines, copiers and the like.
The printhead of an ink jet printer ejects ink through multiple nozzles in the form of miniscule drops which “fly” for a small space and then strike the print media. Typically, different printheads are used for different colors. Ink jet printers usually print within a range of 300 or 2400 or more dots per inch. The ink drops are dried upon the media soon after being deposited to form the desired printed image.
There are several types of ink jet printheads including, for example, thermal printheads and piezoelectric printheads. By way of example, in a thermal ink jet printhead, the ink drops are ejected from individual nozzles by localized heating. Each of the nozzles have a small heating element. Electric current is made to pass through the element to heat it. This causes a tiny volume of ink to be heated and vaporized instantaneously by the heating element. Upon being vaporized, the ink is ejected through the nozzle. Circuitry is connected to the individual heating elements to supply the energy and pulses and, in this manner, to deposit in a controlled way ink drops from associated individual nozzles. These circuits have communications with the imaging circuitry of the printer to activate selected nozzles of the printhead in order to form the desired images on the printing support.
Thermal ink jet printing is based on accurate ballistic delivery of small ink droplets to exact locations on the paper or other media. One key factor for sharp, high quality images stems from the accuracy of the droplet placement. Droplet placement inaccuracies are typically caused by imperfections and variations of the mechanical and geometrical characteristics of the printer and printhead. For example, the defects caused by droplet placement errors appear in a variety of ways and may depend on the printheads being used.
Ink jet printers commonly include a printhead which is mounted on a carriage assembly. The carriage assembly is moveable in a transverse direction, relative to an advance direction of a print medium such as paper. As the printhead is moved across the print medium during a particular pass of the carriage assembly, ink is selectively jetted from dot forming nozzles formed in the printhead and deposited on the print medium at corresponding ink dot placement locations in the image area of the print medium. Since the printhead moves in a direction transverse (e.g., perpendicular) to the advance direction of the print medium, each dot forming nozzle passes in a linear manner over the print medium. The line associated with each dot forming nozzle which overlays the print medium is commonly referred to as a raster or raster line. A plurality of rasters which extend across the image area of the print medium are disposed vertically adjacent to each other in the advance direction of the print medium.
Ink dot placement-related problems vary in severity with a large number of printer-related variables including desired printing speed, print head array configurations, transfer versus direct printing, aqueous versus phase changing, required printing resolution, direction of printing, print post processing, if any, and the type of medium. In particular, color ink jet printing requires careful placement of ink dots to meet current resolution and color fidelity requirements without producing undesired printing artifacts.
The field of ink jet printing is replete with references describing solutions to problems associated with placing ink dots on a print medium. In one known process, a subgroup, which is the same for all current positions at a print line, is formed for a partial number of dot forming nozzles. The dot forming nozzles of the subgroup are selectively controlled at every position according to predetermined print data. Accordingly, depending on the print data of the respective dot forming nozzles, ink may be applied to the recording substrate. After passing across the print line, the recording substrate is advanced in accordance with the length of the subgroup in the forward feed direction. A printhead can then continue to make recordings during the subsequent return movement (bidirectional printing) or only when a new advancing movement of the printhead is effected (unidirectional printing).
Bidirectional printing improves print throughput and is therefore more efficient at a time to print standpoint than unidirectional printing. Unidirectional printing has been used to achieve high quality output in bidirectional capable printers. For example, occasionally print artifacts of bidirectional printing are undesirable in print outcome. The direction chosen for unidirectional printing on a bidirectional capable printer is often predetermined in firmware and/or based on throughput considerations or the proximity of the maintenance station. The assumption that either the left to right carrier direction or the right to left carrier direction will provide an equivalent level of print quality is often erroneous due to asymmetries present in the jetting behavior of the printhead. This problem is further complicated due to manufacturing variations in printheads. Typically, printheads have an optimum print direction that should be used for unidirectional printing to achieve the best quality. In the past, the print direction for unidirectional printing has been determined by the manufacturer in the firmware, and typically is based on a sampling of printheads at the time of manufacture rather than the actual individual printhead(s) in the specific printer. Accordingly, there is a need for a method for determining optimal unidirectional print direction in an ink jet printer.
Manufacturing variations contribute to the tendency of both mono and color printheads to show dot quality differences as a function of carrier direction. Satellite drops, as illustrated in FIG. 1, typically follow the mother drop, and they can land on the medium past the mother drop in the same direction of the carrier motion due to their inherently lower drop velocity. Asymmetrical satellite behavior is very common in manufactured ink jet printheads. The lack of satellite symmetry between left to right jetting versus right to left jetting makes achieving bidirectional dot alignment more challenging. Another difficulty is the lack of symmetry is not consistent from printhead to printhead or manufacture lot to manufacture lot, but inevitably can vary from printhead to printhead.
Satellite asymmetry can cause graininess of a print recording. Graininess in an image will be aggravated by the presence of satellite dots. When a printing system is optimized to achieve all the benefits of unidirectional printing, minimizing graininess should be a high priority. As such, there is a need for a method for determining the optimal direction of carrier travel in which a printhead exhibits the least tendency to generate unwanted satellites while recording an image.